There is considerable and growing interest in high capacity optical interconnects. Interconnects capable of data rates of minimum 40 Gb/s, preferably 100 Gb/s and more are commonly required in various implementations. Due much in part to difficulties in implementing these data rates in a serial format, parallel multichannel single fiber interconnect implementations (i.e., high data capacity systems) are finding favor.
As an example, parallel multichannel interconnect implementations are implied in the 100 Gb/s small-form-factor pluggable (CFP) standard, a multi-source agreement to produce a common form-factor for the transmission of high-speed digital signals. The CFP standard defines a hot-pluggable optical transceiver form factor that enables transmission applications and data rates up to 100 Gb/s, including next-generation High Speed Ethernet (100 GbE). Pluggable CFP transceivers are capable of supporting the ultra-high bandwidth requirements of data communications and telecommunication networks that form the backbone of the Internet. The electrical interfaces of the CFP standard are only generally defined. But parallel links with an array of ten 10 Gb/s devices are favored for a 100 Gb/s transceiver.
Many conventional parallel multichannel array implementations utilize an array of fixed wavelength lasers or integrated laser modulators and operate at a given wavelength band (e.g., C-band or L-band) within an ITU grid (e.g., 200 GHz spacing, 100 GHz spacing, 50 GHz spacing, etc.). That is, each laser of an array may operate at a particular channel and corresponding frequency (wavelength) of an ITU grid. Grid alignment of each of the lasers in the array is accomplished by specific grating pitch, built in the manufacturing process, combined with some thermal adjustment. As an example, the use of Bragg gratings of varying pitch in a multi-wavelength laser emitting component is disclosed in U.S. Pat. No. 5,930,278.
Arrays of directly modulated lasers (DML) are the simplest implementation. For example, the PD100-TX 100 Gb/s CFP compatible fiber optic transceiver from Santur Corporation includes ten independent channels operating at 10.3125 Gb/s per channel. The PD100-TX incorporates a 10 channel laser driver circuit together with a 10 channel DML array and an optical multiplexer.
But arrays consisting of fixed wavelength lasers such as that described above provide limited versatility. As a consequence, many variants of a given fixed wavelength array would be required to meet the requirements of various short (2 km-10 km), medium, and long reach (above 40 km) implementations on various ITU grids. The reach and environment the array is used in can dictate the choice of variants.
For instance, for a 2-10 km short reach, there is little advantage in using dense WDM (Wavelength Division Multiplex) as this would generally be more costly than a coarse WDM multichannel approach. With short links, the fiber represents a small part of the cost, so additional channels (e.g. of 10, 40 or 100 Gb/s) would be added by providing additional fibers. Typically CWDM configuration operate with 4 or 8 nm spacing between the parallel data carrying paths, so the composite set uses up a substantial part of the available transmission band of the fiber and there is little scope for additional wavelength multiplexing to further increase the capacity in the fiber.
For intermediate/long reaches, the fiber cost and installation is much more significant and generally multiple channels (e.g. of 10, 40 or 100 Gb/s) will be required to be transmitted on the fiber. This is accomplished by using dense WDM, where the spacing between the individual parallel data channels will be typically 50, 100 or 200 GHz. In the case of 50 GHz spacing, the traditional ‘C’ band of the fiber between around 1525 and 1565 nm, can carry around 100 lanes of data. For 100 GHz, this is around 50. So in the case of an array for this application, a minimum of around ten different variants with different wavelength outputs would be required for 10 channel units operating on 50 GHz grid and around 25 different variants for 4 channel units operating on a 50 GHz grid, with more variants needed for complete flexibility in the starting channel of the array. As the grid widens (e.g. to 100 GHz), the number of variants reduces in principle, although many users will ‘interleave’ 100 GHz grid devices to use the full 50 GHz grid, giving the same overall number of variants.